Good stuff. Here are a few of the fine points.

Don't expect the new line to fit exactly, the old one always gets bent taking it out. You will have to give the new line a bend here or a twist there to get it to fit, this is normal. Try to take the old line off without bending it too much.

When replacing a single line you can strap it to the old lines with cable ties. This looks crude but is perfectly safe. I prefer not to do it this way for aesthetic reasons but sometimes, well....

For the type of line that has a rubber hose pushed over a steel line, you can duplicate the bulge the factory puts at the end using the flaring tool.

Double flares are best for high pressure like brake lines, probably not necessary for fuel lines. Do whatever the factory did.

On long lines in difficult locations sometimes you need to make it in 2 parts,or for brake lines buy 2 ready made lines to get the length you need. In this case you can buy a connector fitting, where a brake line screws into each end.

For low pressure like fuel lines you can get a similar connector called a compression fitting, it clamps onto a plain line, no flare necessary. Don't use a compression fitting on a brake line, I know people get away with it but it's not rated for high pressure.

Hope this helps.